The invention relates to a tank for liquids and a centrifugal pump for discharging the liquid having a substantially vertical pump shaft, wherein the pump is configured with the intake side at the bottom of the tank and connected, via a drive shaft, to drive means preferably arranged above the tank, wherein the drive shaft with a coupling is in operable connection with the pump shaft, and wherein the pump is, near the coupling, provided with a first main bearing at the upper end of the pump shaft and a second main bearing opposite the coupling at the lower end of the pump shaft near the tank bottom, and wherein a connection is established between the discharge side of the pump step opposite each bearing and the bearing faces of the relevant bearing, and wherein a drain is provided from each bearing to the intake side of the pump step.
Examples of liquid tanks, for which the invention is useful, include the tanks of a tanker, but also other types of tanks whose contents are discharged by means of a pump arranged at the tank bottom are included in the invention. The supply of lubricant, which in this case means pump medium, to each of the main bearings of the pump is accomplished through a passage from the discharge side of the pump step that is situated opposite the relevant bearing to the interior bearing faces of the bearing. In that case each bearing has a drain for lubricant to the intake side of the corresponding pump step. As soon as the pump is operative and there is liquid present in the tank, the lubricant for the bearings will be pressurised.
The problem associated with these pumps occur if they are allowed to run without liquid present in the tank which may occur when the tank is emptied unless the pump is turned off as soon as the tank is empty. When this occurs, in particular those portions of the main bearings that absorb the axial forces are exposed to comprehensive heating, which leads to breakdown.
The components for the axial bearings carry the total weight of the shaft with associated blade wheels, which is a considerable weight, and thereby comprehensive friction heat will be generated unless the bearings are lubricated sufficiently.
In order to remedy this problem the first main bearing near the coupling at the upper end of the pump shaft is a purely radial bearing, and the second main bearing opposite the coupling at the lower end of the pump shaft is a radial and axial bearing.
The pump being only to a small extent auto-adsorbing, residual liquid will always remain in the tank that is not pumped up. However, this residue is sufficient to ensure that the lowermost main bearing at the intake side of the pump is always completely or partially immersed into the liquid. Hereby the external cooling of this bearing is ensured.
Besides, a certain amount of the liquid from the tank bottom is occasionally drawn into the pump, e.g. so much that the liquid level no longer covers the inlet and hereby a liquid pressure will be generated in the passage from the intake side of the first pump step to the bearing surfaces of the lower main bearing with the result that the bearing is lubricated and cooled. When the liquid surface no longer covers the pump inlet the liquid drawn into the pump will return to the tank and a renewed sucking procedure will occur. To a certain extent this will effect lubrication and cooling of the lowermost main bearing such that the axial bearing of the pump is ensured a certain degree of cooling and a certain degree of lubrication. Since simultaneously the upper bearing is a purely radial bearing, it tolerates to a higher degree to run without lubrication and cooling, sincexe2x80x94due to the vertical pump shaft no it is not correspondingly heavily strained.
According to one embodiment of the invention the main bearings have bearing faces of carbon or silicon carbide. Carbon bearings tolerate extremely elevated temperatures and besides carbon bearings contain a certain amount of graphite, which renders the bearing faces auto-lubricating. Bearings with bearing faces of silicon carbide have low friction and simultaneously adequate heat conductivity and thus, on the one hand, the friction heat will not be overwhelming and the heat that is generated is conveyed away by heat conductivity. Besides, both materials tolerate impurities in the lubricant, which will often occur when the pumping medium is used as lubricant.
Particularly conveniently the upper main bearing has bearing faces in carbon and the lower has bearing faces of silicon carbide. Hereby the properties of the two bearing materials are used optimally, since the upper bearing benefits from the auto-lubricating properties and the heat conductivity of the carbon material, and the low friction of the silicon carbide bearing at the bottom ensures low generation of heat and adequate heat conduction.
Tankers can carry many kinds of media, e.g. aggressive and poisonous media or consumption media. The drive shaft is surrounded by a jacket filled with oil or corresponding lubricant to lubricate the bearings of the drive shaft, and it must be ensured both that this lubricant does not pollute the tank contents and that the tank contents do not penetrate into and pollute the lubricant. Therefore the coupling between the drive shaft and the pump shaft is conveniently a magnetic coupler in order to thereby ensure complete separation between the oil-lubricated and the pump-medium-lubricated parts of the construction.
Besides it is a problem that often the liquid in the tank contains impurities that may cause wear to the bearings and in particular if the impurities contain magnetic material, such as e.g. non-oxidised iron in rust particles, they will deposit on the magnetic coupler which will, in the worst case, cause perforation of the membrane that separates the two sides of the magnetic coupler. Therefore, in the connection between the discharge side of a pump step and the bearing faces of the associated main baring, a filter is provided for separation of impurities and preferably a cyclone filter.
Hereby it is ensured that the bearings are not exposed to particles that can cause deposits and wear. This is important in particular to the connection to the upper bearing since the liquid that is supplied here as lubricant also lubricates and cools the area between the interior side of the magnetic coupler and the membrane that separates oil-lubricated and pump-medium-lubricated elements. A filter can be avoided at the lower main bearing, in particular if it is made with a bearing face of a hard material, such as silicon carbide.
A particularly advantageous filter is a cyclone filter since it does not have filter portions that are to be exchanged or cleaned. The residual fraction from the cyclone filter is recycled to the pump during the pump step in which the supply occurs.
The invention also relates to use of a liquid tank and a centrifugal pump according to claim 1 as a storage tank on a tanker. With this use it is accomplished that the discharge of the storage tank becomes an extremely reliable operation and the pump breakdowns that will occur as a consequence of belated interruption of the pump after the discharge operation of the tanker""s storage tanks can largely be avoided.